Field of the Invention
The present invention relates to a thermal processing method and a thermal processing apparatus for heating a precision electronic substrate (hereinafter simply called a “substrate”) in the form of a thin plate such as a semiconductor wafer by irradiating the substrate with a flash.
Description of the Background Art
Impurity introduction performed to form a pn junction in a semiconductor wafer is an essential step in manufacturing process of a semiconductor device. At present, impurities are introduced generally by ion implantation process and subsequent annealing process. The ion implantation process is a technique of implanting impurities physically by ionizing an impurity element such as boron (B), arsenic (As), or phosphorous (P), and making the impurity ions collide with the semiconductor wafer at a highly accelerated voltage. The implanted impurities are activated by the annealing process. If the annealing takes about several seconds or more, the implanted impurities are diffused deeply by heat and a resultant junction reaches a depth greater than is necessary. This might become an obstacle to favorable formation of a device.
Flash lamp annealing (FLA) has attracted attention in recent years as an annealing technique of heating a semiconductor wafer in an extremely short period of time. The flash lamp annealing is a thermal processing technique of increasing the temperature only of a front surface with implanted impurities of a semiconductor wafer in an extremely short period of time (several milliseconds or less) by irradiating the front surface of the semiconductor wafer with a flash using a xenon flash lamp (in the below, a lamp simply called a “flash lamp” means a xenon flash lamp).
The spectral distribution of light emitted from a xenon flash lamp ranges from an ultraviolet region to a near-infrared region, has a shorter wavelength than light from a conventional halogen lamp, and substantially agrees with a base absorption band of a silicon semiconductor wafer. Thus, irradiating the semiconductor wafer with a flash from the xenon flash lamp does not produce much transmitted light, so that the temperature of the semiconductor wafer can be increased rapidly. Additionally, it has become known that irradiation with a flash in an extremely short period of time of several milliseconds or less can increase the temperature only of a front surface and its vicinity of the semiconductor wafer selectively. As a result, increasing a temperature in an extremely short period of time by the xenon flash lamp can realize only activation of impurities without causing deep diffusion of the impurities.
During thermal process not limited to flash heating, what is important is to control the temperature of a semiconductor wafer properly. For such control, the temperature of the semiconductor wafer being processed thermally should be measured accurately. During thermal process on the semiconductor wafer, a temperature is typically measured using a non-contact radiation thermometer. For accurate temperature measurement with the radiation thermometer, the emissivity of a measurement target should be known. Meanwhile, the emissivity of the semiconductor wafer is known to differ largely in a manner that depends on a pattern or a film formed on its front surface. Unless the emissivity is established, temperature measurement with the radiation thermometer is impossible.
According to the suggestion of US 2012/0288970, after the temperature of a front surface and that of a back surface of a semiconductor wafer become equal to each other during irradiation with a flash, the emissivity of the front surface of the wafer is determined based on the temperature of the wafer measured with a radiation thermometer on a side closer to the back surface and light intensity measured on a side closer to the front surface. Then, by using the determined emissivity, the temperature of the front surface of the semiconductor wafer during the irradiation with a flash is determined.
However, according to the technique disclosed in US 2012/0288970, a sensor should be provided for each of the side closer to the front surface and the side closer to the back surface of the semiconductor wafer for measurement. This results in a complicated mechanism and a complicated algorithm for the determination.
Additionally, while a variety of materials have been used for semiconductor purposes in recent years, there has been a strong demand for measuring the temperature of a front surface of a substrate easily where emissivity is very difficult to measure such as a substrate including a silicon base and an epitaxial film of germanium formed on the base, for example.